More particularly, the invention is applicable to depositing an extremely thin coating of a photosensitive resin on the surface of a substrate embodied by any material, especially a semiconducting metal, such as silicon.
In particular, it is known that as regards embodying integrated circuits, assuming the embodiment stage where successive coatings each comprise a conductive network, each stage is embodied by depositing a photosensitive resin on a substrate, which is then insulated with a suitable mask being interposed
followed by fixing so as to embody the network corresponding to the stage involved.
The embodiment of ultra-miniaturized integrated circuits requires depositings of extremely thin coatings of photosensitive resin (referred hereafter as "resin") so as to make it possible to obtain an extremely fine definition as regards each stage.
The technique currently used to obtain a thin coating resin film consists of depositing the liquid resin dissolved in a solvent on the receiving substrate, the latter generally being a circular or four-sided plate.
The plate is firstly deposited and kept on a gyratory plate; the plate is immediately subjected to rotation which, via the effect of centrifugal force, provokes spreading of the extremely thin uniform coating of the liquid constituted by the resin solution and the solvent over the surface of the substrate.
The extreme volatility of the solvent requires that rotation is effected with high accelerations so that correct spreading is obtained before evaporation of the solvent significantly modifies the viscosity of the solvent/resin mixture.
The initial depositing of the dissolved resin may then be effected when the plate is stationary, but it is also possible to deposit the dissolved resin in an initial phase during which the substrate is already slowly rotating, which facilitates the initial dispersion movement over the entire surface fast rotation with subsequent high acceleration is aimed at eliminating from the surface of the substrate the excess resin which is then projected outwardly and then collected on the walls of a bowl inside which the gyratory plate is mounted.
The search for films of increasingly reduced thickness on increasingly large substrates is resulting
in the use of more elaborate techniques respectively acting on the nature of the dissolved resin, especially its concentration and certain properties, especially the volatility of the solvent, as well as acting on the operational conditions, especially the speed of rotation, acceleration intensity, etc. As a reminder, mention is made here of the essential precautions required linked to the perfect purity and stability of the environment, especially by entirely eliminating dust which, once it has settled, would generate an unacceptable source of distortion as regards the required scale of miniaturization.
However, it is clear that the more the size of the substrate is increased whilst decreasing the thickness of the deposited resin film, especially by seeking to obtain film thicknesses of less than one micron, the operational difficulties and conditions become increasingly difficult to implement and control in order to obtain, not only a resin film of reduced thickness, but also an essentially required resin film with a constant and homogeneous thickness.
In effect, the thickness of the film needs to be perfectly even over the entire surface of the substrate.
Apart from the nature (viscosity linked to concentration) of the resin solution, it may be possible to try to reduce the thickness by acting on acceleration and the speed of rotation; thus, accelerations are used able to bring the speed of rotation up to 10,000 revs per minute in one second.
However, experience shows that problems then arise linked to interference between the air and the static gaseous volume located above the substrate and the rotary substrate, as well as the liquid film being fixed and borne by the substrate.
The accelerations thus implemented do not enable a displacement speed gradient of the air (or the gaseous layer) located above the substrate to be established and firstly the interference between the liquid surface driven in rotation during stabilization and hardening and secondly the static or semi-static air causes interference expressed by surface distortions in the form of wrinkles or waves able to be detected by, in particular, optical means.
But in particular the technique thus described becomes totally unsuitable when it is desired to embody spreading of the resin on, for example, four-sided substrates so as to have them exploited as part of embodying flat screens.
In fact, when the four-sided substrate, especially a square-shaped one, is rotated, each angle constitutes a leading edge of the air and consequently generates turbulence which significantly interferes with the spreading and distribution of the liquid film over the surface of the substrate.
As a result, the embodiment of square substrates or plates bearing a photosensitive resin film is consequently accompanied in each angle by a zone revealing traces of turbulences encountered by the liquid when the latter is distributed and resulting in particular in inequalities of the thickness of the resin film expressed in particular by segments of turns marking the angles of the substrate.
Consequently, it currently seems impossible via existing techniques to embody square substrates or plates bearing a thin coating of a resin film of uniform thickness by using the previously described centrifuging and gyratory techniques.